In printing presses, and in particular in rotary printing presses, ink is applied between one or several rollers of an ink unit, between the ink unit and printing group cylinders, possibly between printing group cylinders, and from one printing group cylinder against a counter-pressure cylinder, all of which are referred to as rollers in the following discussion, and to a web, for example a paper web. To this end, the transfer of the ink between two adjoining rollers, which typically cooperate with each other through the web, preferably takes place between a first roller with a “hard” surface and a second roller with a “soft” surface.
Since a certain amount of surface pressure between the rollers is required for transferring the ink, at least the surface of the roller with the soft surface undergoes a deformation in the area of its contact with the hard surfaced roller. This deformation of the soft surface which is structured, for example, as an elastomer dressing, a rubber blanket, a metallic print blanket, sleeve, or coating causes a change in the effective diameter of this roller, with this change depending on the material behavior and the size of a compression, for example as a function of the distance between the rollers, the different thicknesses of the web, etc. In the course of the soft surface roller's rolling off on the cooperating hard roller, this surface deformation leads to changes in the surface pressure and in the roll-off. With rollers which are driven mechanically or which are electronically synchronized, this can lead to different surface velocities as a function of the material used and of the compression, and therefore to slippage in the nip defined as the area of contact between the two rollers.
The slippage generated in this manner results in a tangential force component because of friction, and therefore a reduced print quality which may be due to pushing or blurring, interference with the transfer of power, as well as a reduced service life of printing formes, or dressings.
A print blanket is known from DE 43 15 456 A1, which has an incompressible or dimensionally stabilized carrier layer and a compressible elastomeric layer, wherein the latter increases the tolerances in the printing roll-off. With the use of an optimized layer structure, it can be achieved that in certain contact areas of the cooperating cylinders almost no change in the length of the surfaces occurs. This means that a difference in the angles of rotation of two cylinders rolling off on each other is independent of the compression in these areas. It is possible to determine the difference in the angles of rotation for different dressings and for different contacts by use of a laboratory model, wherein a driven first cylinder and a cylinder which freely runs along and is provided with the dressing, are placed against each other.
A characterizing number for characterizing the roll-off behavior of an elastic dressing is discussed in an article published by the inventors of the subject invention and starting at page 211 of the TAGA Proceedings 2001, and which is entitled “The Effect of Printing Blankets on the Rolling Condition of Printing Cylinders”. This makes it possible for a designer to calculate the transmission ratios between transfer and forme cylinders. A device for determining the roll-off behavior has an externally driven and a friction-driven roller, whose angular velocities can be measured by use of opto-electronic angle decoders.